CPLD ring oscillator

Frank shared a ringoscillator made from a CPLD. It creates a ~18MHz clock with no crystal or external oscillator:

A ringoscillator can be implemented through a shiftregister with an inverted input.

The ringoscillator is feeding a counter and the counter is feeding the two leds.

The most critical part was to prevent xst from optimizing away the ringoscillator. This is achieved by the “keep” syntheses attributes.

Get the CoolRunner-II development board for $15 and XC9572XL dev-board  for $15.

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